I need to be able to replace the value associated with a specific identifier ie. replace the ‘Telephone Number’ with another string $newdata, as this is associated with the identifier ‘a3f9a1ba-85a2-4142-b0d6-6a85bef30cb5’.

You're working with an array of associative arrays stored as a JSON value; hence the json_decode() right there at the beginning that turns your json value into the arrays you're displaying. go head and make your changes like $arr[$index][$key]=$newValue; and assuming you're passing this back, don't forget to json_encode($arr,$flags) beforehand as necessary.